This invention relates generally to air conditioning and refrigeration systems and, more particularly, to a method of oil return to a refrigerant compressor to ensure adequate lubrication of the compressor components and with minimal or no performance degradation of a refrigerant system.
In a vapor compression system such as that used in air conditioners, heat pumps and refrigeration units, refrigerant vapor from an evaporator is drawn in by a compressor, which then delivers the compressed refrigerant to a condenser (or a gas cooler for transcritical applications). In the condenser, heat is exchanged between a secondary fluid such as air or water and the refrigerant, and from the condenser, the refrigerant, typically in a liquid state, passes to an expansion device, where the refrigerant is expanded to a lower pressure and temperature, and then passes to the evaporator. In the air conditioning applications, in the evaporator, heat is exchanged between the refrigerant and another secondary fluid such the indoor air or water to condition the indoor air or to cool water.
Since the refrigerant compressor necessarily involves moving parts, it is typically required to provide lubrication to these parts by means of lubricating oil that is mixed with or entrained in the refrigerant passing through the compressor. Although the lubricant is normally not useful within the system other than in the compressor, its presence in the system does not generally detract from the flow and change of state as the refrigerant passes through the system in a conventional vapor compression cycle. However, there is a tendency for oil to be retained within the evaporator or suction line of the refrigerant system. This is particularly true in a system wherein the evaporator is of a microchannel heat exchanger type and when refrigerant mass flow rates are low. If the oil retention in the evaporator becomes excessive, then the performance of the evaporator, as well as that of the entire system, is degraded due to heat transfer reduction and pressure drop increase. More importantly, the oil retention in the evaporator or suction line may reduce the amount of lubricant passing through the compressor such that it is not adequately lubricated, and damage may occur to the compressor components. In the most severe scenario, all oil can be pumped out of the compressor, leaving the compressor internal elements essentially with no lubrication and leading to quick seizure of the compressor.
One approach to solving this problem is that of providing an oil separator downstream of the compressor such that the oil is removed from the refrigerant prior to passing through the remaining sections of the system. However, an oil separator represents an added expense that is not desirable. Further, oil separators are never 100% efficient, so sooner or later a significant amount of oil may be trapped in the refrigerant system components (other than a compressor) causing abovementioned problems. Oil separators can malfunction (plug up, spring a leak, etc.), would often introduce additional undesirable pressure losses and have an inherent high-to-low pressure refrigerant leak since the oil needs to be returned from a high pressure discharge section back to a low pressure side (normally, a compressor oil sump). Therefore, there is a need for a cost effective method to assure oil return to the compressor that preferably doesn't require any extra components added to a refrigerant system.